While the primary problems giving rise to the invention have arisen from hydraulically powered vehicles and the invention described is particularly suited to such vehicles, it will be recognized that the invention is applicable to a variety of other machines and circumstances. However, inasmuch as the problems have arisen in the field of hydraulically driven vehicles, the invention will be hereinafter discussed as to both problem and solution in that application. The discussion directed to use on hydraulically powered vehicles is for illustrative purposes only and is not intended to be limiting.
The invention is applicable to a variety of ratio changing power transmission requirements, however applied, between a power source and a load and the invention shall be understood accordingly. As the use of hydraulic pumps and hydraulic motors in a power transmission application becomes more common, some of their advantages and disadvantages should be examined. By providing in a simple hydraulic drive system a variable displacement pump and fixed displacement motor, pressure in the system determines motor torque output and volume determines the speed of the motor. Inasmuch as the pump's displacement is variable, it will provide an infinitely variable motor output speed up to its maximum volumetric capacity. However, allowable system pressure, expense, and physical size of hydraulic components impose narrow limits of operation. It is therefore desirable to provide a mechanical ratio changing means which increase the spectrum of operational characteristics.
It is also desirable to effect such ratio changing in a smooth controllable manner and without interrupting the continuous flow of power. As is readily understood, conventional manually shifted gear changing transmissions of the automotive and truck type are totally unsuited to this general application inasmuch as they require a power flow interruption and pass through a "neutral position" during the ratio changing operation.
Interruption of any continuing force present in the power train acting upon the load will either increase or decrease the inertial or static force of that load thus creating a potential load "runaway" or loss of momentum during such power train interruption. A "neutral position" merely magnifies the foregoing problem or hazard in that the uninterrupted mechanical connection to the load must be re-established from the "neutral position" immediately, to control the load. Re-establishing that connection becomes increasingly difficult as the differential speed relationship between the load and the power source broadens.
Further, should hydraulic failure take place in either the hydraulic components or their associated lines, the vehicle or load becomes uncontrollable and must then be arrested by some other means.
Another problem presents itself when, for whatever reason, one attempts to tow a hydraulically powered vehicle or otherwise manually rotate the hydraulic motor output shaft, in that hydraulic circuitry normally will render the motor effectively locked to the pump and then to the engine or power source. It is therefore advantageous to provide a means of effectively disengaging the hydraulic motor from the remaining power train thus enabling one to tow or rotate said power train components independent of the hydraulic drive system.
Accordingly, the objects of the invention are to overcome the aforementioned problems and to provide:
I. a power transmission device capable of effecting a ratio change between a power source and a load without appreciably effecting the flow of power from the source to the load. PA1 Ii. a device, as above mentioned, not having a "neutral position" during the normal ratio changing operation. PA1 Iii. a device, as above mentioned, capable of locking up or arresting a load upon need or demand, such arresting capability being effective without directing any resultant forces through the transmission gear components. PA1 Iv. a device, as above mentioned, providing a means of effectively disengaging the power source from the load as may be required. PA1 V. a device, as above mentioned, having long life while transmitting power at both low and high operational speeds. PA1 1. Rotating pressure seals. PA1 2. Extremely large diameter bearings. PA1 3. High pressure required to effect function. PA1 4. Clutch release bearings which are required to accept continual rotational thrust loads for the full time that selected mode or ratio is in use. PA1 5. Hydraulic control circuitry and valves of a complex nature.
Transmission devices utilizing planetary gear sets comprised of a ring gear, planet gear or gears, a planet gear carrier and a sun gear, being controlled by frictionally engageable clutch means such as disc clutches or bands are generally known and widely used. In transmissions of this general type, one or more of the following undesirable and limiting methods or components are usually present: